To perform color printing, the priming industry has historically used photomechanical processing in which a color photograph is separated into four typical colors by use of color filters. Each of these four colors would then be used in creating a film which is further used to create a plate used in actual four-color priming processes. Conventional printing uses a combination of impression plates and inks applied either directly or through an intermediate level to transfer an image from the plates to the final paper or media. In typical four-color priming, the ink colors used are: yellow, magenta, cyan and black.
The production of the impression plates has historically been achieved through a photomechanical process whereby color filters were used in conjunction with a camera to produce the four "separations" corresponding to the appropriate levels of each color ink in the image to be reproduced. If the final page to be printed included text, the content of the text would be typeset and photographed. The film thereby created would be superimposed by hand onto the black color separation and a new final film shot for the black plate.
With the introduction of sophisticated electronics, much of this process can now be automated. The images (such as photographs) are scanned electronically by a digital imager, which produces data values for each of the four color separations, and these data values can be stored in a computer. The resulting electronic images are sent to a page composition station where an operator places the image and includes any text or other artwork (again by using a computer), to create a format of a final page which is to be printed. Such other artwork can include computer generated images, such as "patterned fills," and "graduated fills," etc.
The final page is now in an electronic format which can be sent to an output device to expose one set of films with all information included. All of the different types of images and other data are combined into a "page description language" file which can be further processed, and has flexibility for making later changes in the page's appearance. One important feature of this electronic format is that it can be edited simply by operation of the computer while observing the prospective final product on the computer's color display. In other words, images can be moved, scaled, and rotated, text can be moved or changed, and computer generated images can be edited, etc., all without any changes made to any physical film or plate.
The above output device typically cannot simply use this electronic format to create films, but must interpret the data in the particular input page description language file before acting upon that data. As the output device interprets the input file, it creates a rasterized file of "on-off" values for the film exposing unit. The device which performs the interpretation is typically called a Raster Image Processor ("RIP"), and the exposure device is often referred to as the "marking engine." The RIP function is typically performed by a "service bureau," which is a company that specializes in taking customers' data (page description language) files and creating the films needed for final color printing production. This process is described in greater detail, hereinbelow.
To create a page description language file, a computer programming language is used to accept commands and data used to specify imaging and other criteria. One such computer programming page description language is PostScript.TM. which can be used include all the above types of images and other data to represent various elements of such images and data that will be used in the printed page. Examples of computer programs that can generate PostScript.TM. code are (1) Adobe Illustrator.TM., (2) Aldus Freehand.TM., (3) Quark XPress.TM., and (4) Aldus Pagemaker.TM.. A PostScript.TM. data file which contains the data used to create the color pages must be converted into bit patterns (or "rasterized"), which is typically performed by a Raster Image Processing device. These bit patterns control the exposing of the film for a particular color by an imagesetter.
The purpose of the rasterizing is to convert all the elements of a page description language file into bitmat pixels before printing. This is required because imagesetters can only print dots on paper or film. Other types of image processing (than rasterization) include applying specific filters to images to create special effects, and rendering (which is a type of rasterization) to create a three-dimensional effect on a two-dimensional page. In addition, certain post-processing steps, such as image enhancement, can be performed upon bitmat pixels before printing. An example of a computer program that performs Raster Image Processing is Adobe CPSI.TM., which is a configurable PostScript.TM. interpreter, and is manufactured by Adobe Systems, Inc. located in Mountain View, Calif.